The programmed elimination of nuclei and other cytoplasmic organelles from cells in the deep cortex of the ocular lens is one of the most striking aspects of terminal differentiation in this tissue. Organelles are light scattering structures and their removal ensures the transparency of the lens substance. The abnormal persistence of organelles is a characteristic feature of cataracts in mice and humans. During the last grant period we completed a morphometric analysis of organelle loss in the developing lens. In the current proposal we will address three related questions that arise directly from these studies: What factors trigger organelle loss? What are the biochemical mechanisms underlying this process and, once the organelles have disappeared, what accounts for the extraordinary longevity of certain mRNAs? In considering possible triggering mechanisms, we will evaluate the role of tissue/matrix interactions and lenticular oxygen gradients. There are some striking similarities between organelle loss in the lens and classical apoptosis. We will determine which elements of the apoptotic pathway are utilized during organelle loss. We will also test the putative involvement of 15-lipoxygenase (an enzyme implicated in erythrocyte organelle loss) in fiber cell denucleation. Finally, our preliminary data indicate that following dissolution of the fiber cell nuclei, specific mRNAs are extraordinarily long lived. We will examine the role of cis- and trans-acting factors in stabilizing one such mRNA, delta crystallin. Together these experiments will provide important new information on lens organelle loss, fiber cell differentiation and cataractogenesis.